1. Field of the Invention
The present invention relates to a semiconductor device suitable for a ferroelectric memory and a method for manufacturing the same.
2. Description of the Related Art
In recent years, along with the advancing digital technology, there is an increasing trend toward large-capacity data processing or storing at high speed. Accordingly, a semiconductor device used for electronic equipment is required to be highly integrated and to exhibit higher performance.
On the back of this, in the semiconductor memory devices, for instance, with an aim to realize higher integration of DRAM, a technology that adopts a ferroelectric material or a high dielectric constant material for a capacitor insulating film of a capacitor element composing the DRAM, as a substitute for a conventional silicon oxide or a silicon nitride, has begun to be researched and developed broadly.
In addition, in order to realize a nonvolatile RAM capable of performing write operation and read operation at low voltage as well as high speed, a technology adopting a ferroelectric film having spontaneous polarization characteristic, as a capacitor insulating film, is also actively researched and developed. Such a semiconductor memory device is called a ferroelectric memory (FeRAM).
The ferroelectric memory stores information by taking advantage of a hysteresis characteristic of the ferroelectric memory. The ferroelectric memory is provided with a ferroelectric capacitor that is configured to have, as a capacitor dielectric film, a ferroelectric film in a manner sandwiched between a pair of electrodes. The ferroelectric film brings about polarization according to applied voltage between the electrodes, and has spontaneous polarization even if the applied voltage is removed. Besides, if the polarity of the applied voltage is inversed, then the polarity of the spontaneous polarization is also inversed. Accordingly, the detection of the spontaneous polarization allows reading out the information. The ferroelectric memory operates at a voltage lower than that for operating a flash memory, enabling a power-saving and high-speed write operation.
It should be noted that, when manufacturing a ferroelectric capacitor, a thermal treatment under an oxygen environment needs to be conducted two times or more to recover damage caused in the ferroelectric film. Therefore, as a material for an upper electrode, a metal being difficult to be oxidized even under the oxygen environment, such as platinum or the like, or otherwise a conductive oxide such as IrOx, RuOx, or the like, is employed.
In APPL. Phys. Lett. 65, P.19 (1994) (Non-patent document 1), there is a description saying that, as a material of an upper electrode and a lower electrode sandwiching a ferroelectric film formed of PZT (Pb (Zr, Ti) O3), the use of iridium oxide (IrO2) can prevent the ferroelectric capacitor from fatigue to thereby ensure favorable capacitance characteristics. However, it is known that, with the use of the IrO2 electrode, an abnormally grown large crystal formed of IrO2 is apt to be generated on the surface of the electrode. Such a large crystal causes to produce a defect, degrades electric characteristics of the ferroelectric capacitor, and eventually lowers the yield of the semiconductor device.
Further, in Japanese Patent Application Laid-Open No. 2001-127262 (Patent document 1), for the purpose of solving the above-stated problem, there is disclosed a two-step sputtering method, in which an IrO2 film is formed at low power and further an IrO2 film is formed at high power thereafter. In Japanese Patent Application Laid-Open No. 2000-91270 (Patent document 2), for the same purpose, there is disclosed a method, in which an Ir film and an IrO2 film are formed in a row. Further, for reducing holes in the ferroelectric film, there is also disclosed a method in which an IrO2 film is formed followed by a RTA (Rapid Thermal Annealing) and further an Ir film is formed.
Similar to the other semiconductor devices, the ferroelectric memories are also demanded a miniaturization, low-voltage operation, and so forth. In the ferroelectric memory manufactured by the previously-described conventional method, however, the decline in the polarization inversion amount (switching charge amount) Qsw, which is caused together with the ferroelectric film being thinned, becomes more significant, and at the same time the coercive voltage becomes difficult to decline. When the polarization inversion amount declines, the ferroelectric memory becomes difficult to operate at low voltage, and when the coercive voltage is difficult to decline, the speed of inverting polarity becomes difficult to be improved.